The
CambrianCambrian explosion or
CambrianCambrian radiation[1] was an event
approximately 541 million years ago in the
CambrianCambrian period when
most major animal phyla appeared in the fossil record.[2][3] It lasted
for about 20[4][5]–25[6][7] million years. It resulted in the
divergence of most modern metazoan phyla.[8] The event was accompanied
by major diversification of other organisms.[note 1]
Before the
CambrianCambrian explosion,[note 2] most organisms were simple,
composed of individual cells occasionally organized into colonies.
Over the following 70 to 80 million years, the rate of diversification
accelerated, and the variety of life began to resemble that of
today.[10] Almost all present animal phyla appeared during this
period.[11][12]
The
CambrianCambrian explosion has generated extensive scientific debate.

Main article: Evolutionary history of life
The seemingly rapid appearance of fossils in the "Primordial Strata"
was noted by
William BucklandWilliam Buckland in the 1840s,[13] and in his 1859 book
On the Origin of Species,
Charles DarwinCharles Darwin discussed the then
inexplicable lack of earlier fossils as one of the main difficulties
for his theory of descent with slow modification through natural
selection.[14] The long-running puzzlement about the appearance of the
CambrianCambrian fauna, seemingly abruptly, without precursor, centers on
three key points: whether there really was a mass diversification of
complex organisms over a relatively short period of time during the
early Cambrian; what might have caused such rapid change; and what it
would imply about the origin of animal life. Interpretation is
difficult due to a limited supply of evidence, based mainly on an
incomplete fossil record and chemical signatures remaining in Cambrian
rocks.
The first discovered
CambrianCambrian fossils were trilobites, described by
Edward Lhuyd, the curator of Oxford Museum, in 1698.[15] Although
their evolutionary importance was not known, on the basis of their old
age,
William BucklandWilliam Buckland (1784–1856) realised that a dramatic
step-change in the fossil record had occurred around the base of what
we now call the Cambrian.[13] Nineteenth-century geologists such as
Adam SedgwickAdam Sedgwick and
Roderick MurchisonRoderick Murchison used the fossils for dating rock
strata, specifically for establishing the
CambrianCambrian and Silurian
periods.[16] By 1859, leading geologists including Roderick Murchison,
were convinced that what was then called the lowest
SilurianSilurian stratum
showed the origin of life on Earth, though others, including Charles
Lyell, differed. In On the Origin of Species, Charles Darwin
considered this sudden appearance of a solitary group of trilobites,
with no apparent antecedents, and absence of other fossils, to be
"undoubtedly of the gravest nature" among the difficulties in his
theory of natural selection. He reasoned that earlier seas had swarmed
with living creatures, but that their fossils had not been found due
to the imperfections of the fossil record.[14] In the sixth edition of
his book, he stressed his problem further as:[17]

To the question why we do not find rich fossiliferous deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer.

American paleontologist Charles Walcott, who studied the Burgess Shale
fauna, proposed that an interval of time, the "Lipalian", was not
represented in the fossil record or did not preserve fossils, and that
the ancestors of the
CambrianCambrian animals evolved during this time.[18]
Earlier fossil evidence has since been found. The earliest claim is
that the history of life on earth goes back 3,850 million years:[19]
Rocks of that age at Warrawoona, Australia, were claimed to contain
fossil stromatolites, stubby pillars formed by colonies of
microorganisms. Fossils (Grypania) of more complex eukaryotic cells,
from which all animals, plants, and fungi are built, have been found
in rocks from 1,400 million years ago, in
ChinaChina and Montana.
Rocks dating from 580 to 543 million years ago contain
fossils of the Ediacara biota, organisms so large that they are likely
multicelled, but very unlike any modern organism.[20] In 1948, Preston
Cloud argued that a period of "eruptive" evolution occurred in the
Early Cambrian,[21] but as recently as the 1970s, no sign was seen of
how the 'relatively' modern-looking organisms of the Middle and Late
CambrianCambrian arose.[20]

The intense modern interest in this "
CambrianCambrian explosion" was sparked
by the work of
Harry B. Whittington and colleagues, who, in the 1970s,
reanalysed many fossils from the
Burgess ShaleBurgess Shale and concluded that
several were as complex as, but different from, any living
animals.[22][23] The most common organism, Marrella, was clearly an
arthropod, but not a member of any known arthropod class. Organisms
such as the five-eyed
OpabiniaOpabinia and spiny slug-like
WiwaxiaWiwaxia were so
different from anything else known that Whittington's team assumed
they must represent different phyla, seemingly unrelated to anything
known today. Stephen Jay Gould's popular 1989 account of this work,
Wonderful Life,[24] brought the matter into the public eye and raised
questions about what the explosion represented. While differing
significantly in details, both Whittington and Gould proposed that all
modern animal phyla had appeared almost simultaneously in a rather
short span of geological period. This view led to the modernization of
Darwin's tree of life and the theory of punctuated equilibrium, which
Eldredge and Gould developed in the early 1970s and which views
evolution as long intervals of near-stasis "punctuated" by short
periods of rapid change.[25]
Other analyses, some more recent and some dating back to the 1970s,
argue that complex animals similar to modern types evolved well before
the start of the Cambrian.[26][27][28]
Dating the Cambrian[edit]
Radiometric dates for much of the Cambrian, obtained by analysis of
radioactive elements contained within rocks, have only recently become
available, and for only a few regions.
Relative dating (A was before B) is often assumed sufficient for
studying processes of evolution, but this, too, has been difficult,
because of the problems involved in matching up rocks of the same age
across different continents.[29]
Therefore, dates or descriptions of sequences of events should be
regarded with some caution until better data become available.
Body fossils[edit]
Fossils of organisms' bodies are usually the most informative type of
evidence. Fossilization is a rare event, and most fossils are
destroyed by erosion or metamorphism before they can be observed.
Hence, the fossil record is very incomplete, increasingly so as
earlier times are considered. Despite this, they are often adequate to
illustrate the broader patterns of life's history.[30] Also, biases
exist in the fossil record: different environments are more favourable
to the preservation of different types of organism or parts of
organisms.[31] Further, only the parts of organisms that were already
mineralised are usually preserved, such as the shells of molluscs.
Since most animal species are soft-bodied, they decay before they can
become fossilised. As a result, although 30-plus phyla of living
animals are known, two-thirds have never been found as fossils.[20]

The
CambrianCambrian fossil record includes an unusually high number of
lagerstätten, which preserve soft tissues. These allow
paleontologists to examine the internal anatomy of animals, which in
other sediments are only represented by shells, spines, claws,
etc. – if they are preserved at all. The most significant
CambrianCambrian lagerstätten are the early
CambrianCambrianMaotianshan shaleMaotianshan shale beds
of Chengjiang (Yunnan, China) and
Sirius Passet (Greenland);[32] the
middle
CambrianCambrianBurgess ShaleBurgess Shale (British Columbia, Canada);[33] and the
late
CambrianCambrianOrstenOrsten (Sweden) fossil beds.
While lagerstätten preserve far more than the conventional fossil
record, they are far from complete. Because lagerstätten are
restricted to a narrow range of environments (where soft-bodied
organisms can be preserved very quickly, e.g. by mudslides), most
animals are probably not represented; further, the exceptional
conditions that create lagerstätten probably do not represent normal
living conditions.[34] In addition, the known
CambrianCambrian lagerstätten
are rare and difficult to date, while Precambrian lagerstätten have
yet to be studied in detail.
The sparseness of the fossil record means that organisms usually exist
long before they are found in the fossil record – this is known
as the Signor–Lipps effect.[35]
Trace fossils[edit]

Trace fossils consist mainly of tracks and burrows, but also include
coprolites (fossil feces) and marks left by feeding.[36][37] Trace
fossils are particularly significant because they represent a data
source that is not limited to animals with easily fossilized hard
parts, and reflects organisms' behaviour. Also, many traces date from
significantly earlier than the body fossils of animals that are
thought to have been capable of making them.[38] While exact
assignment of trace fossils to their makers is generally impossible,
traces may, for example, provide the earliest physical evidence of the
appearance of moderately complex animals (comparable to
earthworms).[37]
GeochemicalGeochemical observations[edit]
Main article: Early
CambrianCambrian geochemical fluctuations
Several chemical markers indicate a drastic change in the environment
around the start of the Cambrian. The markers are consistent with a
mass extinction,[39][40] or with a massive warming resulting from the
release of methane ice.[41] Such changes may reflect a cause of the
CambrianCambrian explosion, although they may also have resulted from an
increased level of biological activity – a possible result of
the explosion.[41] Despite these uncertainties, the geochemical
evidence helps by making scientists focus on theories that are
consistent with at least one of the likely environmental changes.
Phylogenetic techniques[edit]
CladisticsCladistics is a technique for working out the "family tree" of a set
of organisms. It works by the logic that, if groups B and C have more
similarities to each other than either has to group A, then B and C
are more closely related to each other than either is to A.
Characteristics that are compared may be anatomical, such as the
presence of a notochord, or molecular, by comparing sequences of DNA
or protein. The result of a successful analysis is a hierarchy of
clades – groups whose members are believed to share a common
ancestor. The cladistic technique is sometimes problematic, as some
features, such as wings or camera eyes, evolved more than once,
convergently – this must be taken into account in analyses.
From the relationships, it may be possible to constrain the date that
lineages first appeared. For instance, if fossils of B or C date to
X million years ago and the calculated "family tree" says A was
an ancestor of B and C, then A must have evolved more than
X million years ago.
It is also possible to estimate how long ago two living clades
diverged – i.e. about how long ago their last common ancestor
must have lived – by assuming that
DNADNA mutations accumulate at
a constant rate. These "molecular clocks", however, are fallible, and
provide only a very approximate timing: they are not sufficiently
precise and reliable for estimating when the groups that feature in
the
CambrianCambrian explosion first evolved,[42] and estimates produced by
different techniques vary by a factor of two.[43] However, the clocks
can give an indication of branching rate, and when combined with the
constraints of the fossil record, recent clocks suggest a sustained
period of diversification through the
EdiacaranEdiacaran and Cambrian.[44]
Explanation of key scientific terms[edit]

Phylum[edit]
A phylum is the highest level in the Linnaean system for classifying
organisms. Phyla can be thought of as groupings of animals based on
general body plan.[46] Despite the seemingly different external
appearances of organisms, they are classified into phyla based on
their internal and developmental organizations.[47] For example,
despite their obvious differences, spiders and barnacles both belong
to the phylum Arthropoda, but earthworms and tapeworms, although
similar in shape, belong to different phyla. As chemical and genetic
testing becomes more accurate, previously hypothesised phyla are often
entirely reworked.
A phylum is not a fundamental division of nature, such as the
difference between electrons and protons. It is simply a very
high-level grouping in a classification system created to describe all
currently living organisms. This system is imperfect, even for modern
animals: different books quote different numbers of phyla, mainly
because they disagree about the classification of a huge number of
worm-like species. As it is based on living organisms, it accommodates
extinct organisms poorly, if at all.[20][48]
Stem group[edit]
The concept of stem groups was introduced to cover evolutionary
"aunts" and "cousins" of living groups, and have been hypothesized
based on this scientific theory. A crown group is a group of closely
related living animals plus their last common ancestor plus all its
descendants. A stem group is a set of offshoots from the lineage at a
point earlier than the last common ancestor of the crown group; it is
a relative concept, for example tardigrades are living animals that
form a crown group in their own right, but Budd (1996) regarded them
as also being a stem group relative to the arthropods.[45][49]

A coelomate animal is basically a set of concentric tubes, with a gap
between the gut and the outer tubes.

Triploblastic[edit]
The term
Triploblastic means consisting of three layers, which are
formed in the embryo, quite early in the animal's development from a
single-celled egg to a larva or juvenile form. The innermost layer
forms the digestive tract (gut); the outermost forms skin; and the
middle one forms muscles and all the internal organs except the
digestive system. Most types of living animal are
triploblastic – the best-known exceptions are Porifera
(sponges) and
CnidariaCnidaria (jellyfish, sea anemones, etc.).
Bilaterian[edit]
The bilaterians are animals that have right and left sides at some
point in their life histories. This implies that they have top and
bottom surfaces and, importantly, distinct front and back ends. All
known bilaterian animals are triploblastic, and all known
triploblastic animals are bilaterian. Living echinoderms (sea stars,
sea urchins, sea cucumbers, etc.) 'look' radially symmetrical (like
wheels) rather than bilaterian, but their larvae exhibit bilateral
symmetry and some of the earliest echinoderms may have been
bilaterally symmetrical.[50]
PoriferaPorifera and
CnidariaCnidaria are radially
symmetrical, not bilaterian, and not triploblastic.
Coelomate[edit]
The term
CoelomateCoelomate means having a body cavity (coelom) containing the
internal organs. Most of the phyla featured in the debate about the
CambrianCambrian explosion are coelomates: arthropods, annelid worms,
molluscs, echinoderms, and chordates – the noncoelomate
priapulids are an important exception. All known coelomate animals are
triploblastic bilaterians, but some triploblastic bilaterian animals
do not have a coelom – for example flatworms, whose organs are
surrounded by unspecialized tissues.
Precambrian life[edit]
Understanding of the
CambrianCambrian explosion relies upon knowing what was
there beforehand – did the event herald the sudden appearance
of a wide range of animals and behaviours, or did such things exist
beforehand?
Phylogenetic analysisPhylogenetic analysis has been used to support the view that during
the
CambrianCambrian explosion, metazoans (multi-celled animals) evolved
monophyletically from a single common ancestor: flagellated colonial
protists similar to modern choanoflagellates.
Evidence of animals around 1 billion years ago[edit]

Modern stromatolites in Hamelin Pool Marine Nature Reserve, Western
Australia

Changes in the abundance and diversity of some types of fossil have
been interpreted as evidence for "attacks" by animals or other
organisms. Stromatolites, stubby pillars built by colonies of
microorganisms, are a major constituent of the fossil record from
about 2,700 million years ago, but their abundance and diversity
declined steeply after about 1,250 million years ago. This
decline has been attributed to disruption by grazing and burrowing
animals.[26][27][51]
Precambrian marine diversity was dominated by small fossils known as
acritarchs. This term describes almost any small organic walled fossil
– from the egg cases of small metazoans to resting cysts of many
different kinds of green algae. After appearing around
2,000 million years ago, acritarchs underwent a boom around
1,000 million years ago, increasing in abundance, diversity,
size, complexity of shape, and especially size and number of spines.
Their increasingly spiny forms in the last 1 billion years may
indicate an increased need for defence against predation. Other groups
of small organisms from the
Neoproterozoic era also show signs of
antipredator defenses.[51] A consideration of taxon longevity appears
to support an increase in predation pressure around this time.[52] In
general, the fossil record shows a very slow appearance of these
lifeforms in the Precambrian, with many cyanobacterial species making
up much of the underlying sediment.[53]
Fossils of the Doushantuo formation[edit]
Main article: Doushantuo formation
The layers of the
Doushantuo formation from around 580 million year
old[54] harbour microscopic fossils that may represent early
bilaterians. Some have been described as animal embryos and eggs,
although some may represent the remains of giant bacteria.[55] Another
fossil, Vernanimalcula, has been interpreted as a coelomate
bilaterian,[56] but may simply be an infilled bubble.[57]
These fossils form the earliest hard-and-fast evidence of animals, as
opposed to other predators.[55][58]
Burrows[edit]
Main article:
CambrianCambrian substrate revolution

The traces of organisms moving on and directly underneath the
microbial mats that covered the
EdiacaranEdiacaran sea floor are preserved from
the
EdiacaranEdiacaran period, about 565 million years ago.[note 3] They
were probably made by organisms resembling earthworms in shape, size,
and how they moved. The burrow-makers have never been found preserved,
but, because they would need a head and a tail, the burrowers probably
had bilateral symmetry – which would in all probability make
them bilaterian animals.[61] They fed above the sediment surface, but
were forced to burrow to avoid predators.[62]
Around the start of the
CambrianCambrian (about 542 million years ago),
many new types of traces first appear, including well-known vertical
burrows such as
DiplocraterionDiplocraterion and Skolithos, and traces normally
attributed to arthropods, such as
CruzianaCruziana and Rusophycus. The
vertical burrows indicate that worm-like animals acquired new
behaviours, and possibly new physical capabilities. Some Cambrian
trace fossils indicate that their makers possessed hard exoskeletons,
although they were not necessarily mineralised.[60]
Burrows provide firm evidence of complex organisms; they are also much
more readily preserved than body fossils, to the extent that the
absence of trace fossils has been used to imply the genuine absence of
large, motile, bottom-dwelling organisms.[citation needed] They
provide a further line of evidence to show that the
CambrianCambrian explosion
represents a real diversification, and is not a preservational
artefact.[63]
Indeed, as burrowing became established, it allowed an explosion of
its own, for as burrowers disturbed the sea floor, they aerated it,
mixing oxygen into the toxic muds. This made the bottom sediments more
hospitable, and allowed a wider range of organisms to inhabit
them – creating new niches and the scope for higher
diversity.[63]
EdiacaranEdiacaran organisms[edit]

Main articles: Ediacara biota, Cloudinid, Kimberella, and Spriggina
At the start of the
EdiacaranEdiacaran period, much of the acritarch fauna,
which had remained relatively unchanged for hundreds of millions of
years, became extinct, to be replaced with a range of new, larger
species, which would prove far more ephemeral.[53] This radiation, the
first in the fossil record,[53] is followed soon after by an array of
unfamiliar, large, fossils dubbed the Ediacara biota,[64] which
flourished for 40 million years until the start of the Cambrian.[65]
Most of this "Ediacara biota" were at least a few centimeters long,
significantly larger than any earlier fossils. The organisms form
three distinct assemblages, increasing in size and complexity as time
progressed.[66]
Many of these organisms were quite unlike anything that appeared
before or since, resembling discs, mud-filled bags, or quilted
mattresses – one palæontologist proposed that the strangest
organisms should be classified as a separate kingdom, Vendozoa.[67]

Fossil of Kimberella, a triploblastic bilaterian, and possibly a
mollusc

At least some may have been early forms of the phyla at the heart of
the "
CambrianCambrian explosion" debate, having been interpreted as early
molluscs (Kimberella),[28][68] echinoderms (Arkarua);[69] and
arthropods (Spriggina,[70] Parvancorina).[71] Still, debate exists
about the classification of these specimens, mainly because the
diagnostic features that allow taxonomists to classify more recent
organisms, such as similarities to living organisms, are generally
absent in the ediacarans.[72] However, there seems little doubt that
KimberellaKimberella was at least a triploblastic bilaterian animal.[72] These
organisms are central to the debate about how abrupt the Cambrian
explosion was. If some were early members of the animal phyla seen
today, the "explosion" looks a lot less sudden than if all these
organisms represent an unrelated "experiment", and were replaced by
the animal kingdom fairly soon thereafter (40M years is "soon" by
evolutionary and geological standards).
Beck Spring Dolomite[edit]
Paul Knauth, a geologist at Arizona State University, maintains that
photosynthesizing organisms such as algae may have grown over a 750-
to 800-million-year-old formation in
Death ValleyDeath Valley known as the Beck
Spring Dolomite. In the early 1990s, samples from this 1,000-foot
thick layer of dolomite revealed that the region housed flourishing
mats of photosynthesizing, unicellular life forms which antedated the
CambrianCambrian explosion.
Microfossils have been unearthed from holes riddling the otherwise
barren surface of the dolomite. These geochemical and microfossil
findings support the idea that during the Precambrian period, complex
life evolved both in the oceans and on land. Knauth contends that
animals may well have had their origins in freshwater lakes and
streams, and not in the oceans.
Some 30 years later, a number of studies have documented an abundance
of geochemical and microfossil evidence showing that life covered the
continents as far back as 2.2 billion years ago. Many paleobiologists
now accept the idea that simple life forms existed on land during the
Precambrian, but are opposed to the more radical idea that
multicellular life thrived on land more than 600 million years
ago.[73]
Ediacaran–Early
CambrianCambrian skeletonisation[edit]
The first
EdiacaranEdiacaran and lowest
CambrianCambrian (Nemakit-Daldynian) skeletal
fossils represent tubes and problematic sponge spicules.[74] The
oldest sponge spicules are monaxon siliceous, aged around
580 million years ago, known from the Doushantou Formation in
ChinaChina and from deposits of the same age in Mongolia, although the
interpretation of these fossils as spicules has been challenged.[75]
In the late Ediacaran-lowest Cambrian, numerous tube dwellings of
enigmatic organisms appeared. It was organic-walled tubes (e.g.
Saarina) and chitinous tubes of the sabelliditids (e.g. Sokoloviina,
Sabellidites, Paleolina)[76][77] that prospered up to the beginning of
the Tommotian. The mineralized tubes of Cloudina, Namacalathus,
Sinotubulites, and a dozen more of the other organisms from carbonate
rocks formed near the end of the
EdiacaranEdiacaran period from
549 to 542 million years ago, as well as the triradially
symmetrical mineralized tubes of anabaritids (e.g. Anabarites,
Cambrotubulus) from uppermost
EdiacaranEdiacaran and lower Cambrian.[78]
EdiacaranEdiacaran mineralized tubes are often found in carbonates of the
stromatolite reefs and thrombolites,[79][80] i.e. they could live in
an environment adverse to the majority of animals.
Although they are as hard to classify as most other Ediacaran
organisms, they are important in two other ways. First, they are the
earliest known calcifying organisms (organisms that built shells from
calcium carbonate).[80][81][82] Secondly, these tubes are a device to
rise over a substrate and competitors for effective feeding and, to a
lesser degree, they serve as armor for protection against predators
and adverse conditions of environment. Some
CloudinaCloudina fossils show
small holes in shells. The holes possibly are evidence of boring by
predators sufficiently advanced to penetrate shells.[83] A possible
"evolutionary arms race" between predators and prey is one of the
hypotheses that attempt to explain the
CambrianCambrian explosion.[51]
In the lowest Cambrian, the stromatolites were decimated. This allowed
animals to begin colonization of warm-water pools with carbonate
sedimentation. At first, it was anabaritids and
Protohertzina (the
fossilized grasping spines of chaetognaths) fossils. Such mineral
skeletons as shells, sclerites, thorns, and plates appeared in
uppermost Nemakit-Daldynian; they were the earliest species of
halkierids, gastropods, hyoliths and other rare organisms. The
beginning of the
Tommotian has historically been understood to mark an
explosive increase of the number and variety of fossils of molluscs,
hyoliths, and sponges, along with a rich complex of skeletal elements
of unknown animals, the first archaeocyathids, brachiopods,
tommotiids, and others.[84][85][86][87] This sudden increase is
partially an artefact of missing strata at the
Tommotian type section,
and most of this fauna in fact began to diversify in a series of
pulses through the
Nemakit-DaldynianNemakit-Daldynian and into the Tommotian.[88]
Some animals may already have had sclerites, thorns, and plates in the
EdiacaranEdiacaran (e.g.
KimberellaKimberella had hard sclerites, probably of carbonate),
but thin carbonate skeletons cannot be fossilized in siliciclastic
deposits.[89] Older (~750 Ma) fossils indicate that
mineralization long preceded the Cambrian, probably defending small
photosynthetic algae from single-celled eukaryotic predators.[90][91]
CambrianCambrian life[edit]
Trace fossils[edit]
Trace fossils (burrows, etc.) are a reliable indicator of what life
was around, and indicate a diversification of life around the start of
the Cambrian, with the freshwater realm colonized by animals almost as
quickly as the oceans.[92]
Small shelly fauna[edit]
Main article: Small shelly fauna
Fossils known as "small shelly fauna" have been found in many parts on
the world, and date from just before the
CambrianCambrian to about 10 million
years after the start of the
CambrianCambrian (the
Nemakit-DaldynianNemakit-Daldynian and
Tommotian ages; see timeline). These are a very mixed collection of
fossils: spines, sclerites (armor plates), tubes, archeocyathids
(sponge-like animals), and small shells very like those of brachiopods
and snail-like molluscs – but all tiny, mostly 1 to 2 mm
long.[93]
While small, these fossils are far more common than complete fossils
of the organisms that produced them; crucially, they cover the window
from the start of the
CambrianCambrian to the first lagerstätten: a period of
time otherwise lacking in fossils. Hence, they supplement the
conventional fossil record and allow the fossil ranges of many groups
to be extended.
Early
CambrianCambrian trilobites and echinoderms[edit]

A fossilized trilobite, an ancient type of arthropod: This specimen,
from the Burgess Shale, preserves "soft parts" – the antennae
and legs.

The earliest trilobite fossils are about 530 million years old, but
the class was already quite diverse and worldwide, suggesting they had
been around for quite some time.[94] The fossil record of trilobites
began with the appearance of trilobites with mineral exoskeletons –
not from the time of their origin.
The earliest generally accepted echinoderm fossils appeared a little
bit later, in the Late Atdabanian; unlike modern echinoderms, these
early
CambrianCambrian echinoderms were not all radially symmetrical.[95]
These provide firm data points for the "end" of the explosion, or at
least indications that the crown groups of modern phyla were
represented.
Burgess ShaleBurgess Shale type faunas[edit]
Main article:
Burgess ShaleBurgess Shale type preservation
The
Burgess ShaleBurgess Shale and similar lagerstätten preserve the soft parts of
organisms, which provide a wealth of data to aid in the classification
of enigmatic fossils. It often preserved complete specimens of
organisms only otherwise known from dispersed parts, such as loose
scales or isolated mouthparts. Further, the majority of organisms and
taxa in these horizons are entirely soft-bodied, hence absent from the
rest of the fossil record.[96] Since a large part of the ecosystem is
preserved, the ecology of the community can also be tentatively
reconstructed.[verification needed] However, the assemblages may
represent a "museum": a deep-water ecosystem that is evolutionarily
"behind" the rapidly diversifying fauna of shallower waters.[97]
Because the lagerstätten provide a mode and quality of preservation
that is virtually absent outside of the Cambrian, many organisms
appear completely different from anything known from the conventional
fossil record. This led early workers in the field to attempt to
shoehorn the organisms into extant phyla; the shortcomings of this
approach led later workers to erect a multitude of new phyla to
accommodate all the oddballs. It has since been realised that most
oddballs diverged from lineages before they established the phyla
known today[clarification needed] – slightly different designs,
which were fated to perish rather than flourish into phyla, as their
cousin lineages did.
The preservational mode is rare in the preceding
EdiacaranEdiacaran period, but
those assemblages known show no trace of animal life – perhaps
implying a genuine absence of macroscopic metazoans.[98]
Early
CambrianCambrian crustaceans[edit]
Further information: Orsten
Crustaceans, one of the four great modern groups of arthropods, are
very rare throughout the Cambrian. Convincing crustaceans were once
thought to be common in Burgess Shale-type biotas, but none of these
individuals can be shown to fall into the crown group of "true
crustaceans".[99] The
CambrianCambrian record of crown-group crustaceans comes
from microfossils. The Swedish
OrstenOrsten horizons contain later Cambrian
crustaceans, but only organisms smaller than 2 mm are preserved.
This restricts the data set to juveniles and miniaturised adults.
A more informative data source is the organic microfossils of the
Mount Cap formation, Mackenzie Mountains, Canada. This late Early
CambrianCambrian assemblage (510 to 515 million years ago) consists
of microscopic fragments of arthropods' cuticle, which is left behind
when the rock is dissolved with hydrofluoric acid. The diversity of
this assemblage is similar to that of modern crustacean faunas. Most
interestingly, analysis of fragments of feeding machinery found in the
formation shows that it was adapted to feed in a very precise and
refined fashion. This contrasts with most other early Cambrian
arthropods, which fed messily by shovelling anything they could get
their feeding appendages on into their mouths. This sophisticated and
specialised feeding machinery belonged to a large (about
30 cm)[100] organism, and would have provided great potential for
diversification; specialised feeding apparatus allows a number of
different approaches to feeding and development, and creates a number
of different approaches to avoid being eaten.[99]
Early
OrdovicianOrdovician radiation[edit]
Main article:
OrdovicianOrdovician radiation
After an extinction at the Cambrian–
OrdovicianOrdovician boundary, another
radiation occurred, which established the taxa that would dominate the
Palaeozoic.[101]
During this radiation, the total number of orders doubled, and
families tripled,[101] increasing marine diversity to levels typical
of the Palaeozoic,[41] and disparity to levels approximately
equivalent to today's.[10]
Stages[edit]
The event lasted for about the next 20[102][103]–25[104][105]
million years. Different authors break the explosion down into stages
in different ways.
Ed Landing recognizes three stages: Stage 1, spanning the
Ediacaran-
CambrianCambrian boundary, corresponds to a diversification of
biomineralizing animals and of deep and complex burrows; Stage 2,
corresponding to the radiation of molluscs and stem-group Brachiopods
(hyoliths and tommotiids), which apparently arose in intertidal
waters; and Stage 3, seeing the
Atdabanian diversification of
trilobites in deeper waters, but little change in the intertidal
realm.[106]
Graham Budd synthesises various schemes to produce a compatible view
of the SSF record of the
CambrianCambrian explosion, divided slightly
differently into four intervals: a "Tube world", lasting from
550 to 536 million years ago, spanning the
Ediacaran-
CambrianCambrian boundary, dominated by Cloudina,
Namacalathus ans
pseudoconodont-type element; a "Sclerite world", seeing the rise of
halkieriids, tommotiids, and hyoliths, lasting to the end of the
FortunianFortunian (c. 525 Ma); a brachiopod world, perhaps corresponding to
the as yet unratified
CambrianCambrian Stage 2; and
TrilobiteTrilobite World, kicking
off in Stage 3.[107]
Complementary to the shelly fossil record, trace fossils can be
divided into five subdivisions: "Flat world" (late Ediacaran), with
traces restricted to the sediment surface; Protreozoic III (after
Jensen), with increasing complexity; pedum world, initiated at the
base of the
CambrianCambrian with the base of the T.pedum zone (see discussion
at Cambrian#Dating the Cambrian);
RusophycusRusophycus world, spanning
536 to 521 million years ago and thus corresponding exactly
to the periods of Sclerite World and
BrachiopodBrachiopod World under the SSF
paradigm; and
CruzianaCruziana world, with an obvious correspondence to
TrilobiteTrilobite World. [107]
Validity[edit]
There is strong evidence for species of
CnidariaCnidaria and
PoriferaPorifera existing
in the Ediacaran[108] and possible members of
PoriferaPorifera even before
that during the Cryogenian.[109] Bryozoans don't appear in the fossil
record until after the Cambrian, in the Lower Ordovician.[110]
The fossil record as Darwin knew it seemed to suggest that the major
metazoan groups appeared in a few million years of the early to
mid-Cambrian, and even in the 1980s, this still appeared to be the
case.[23][24]
However, evidence of Precambrian Metazoa is gradually accumulating. If
the
EdiacaranEdiacaranKimberellaKimberella was a mollusc-like protostome (one of the two
main groups of coelomates),[28][68] the protostome and deuterostome
lineages must have split significantly before 550 million years
ago (deuterostomes are the other main group of coelomates).[111] Even
if it is not a protostome, it is widely accepted as a
bilaterian.[72][111] Since fossils of rather modern-looking cnidarians
(jellyfish-like organisms) have been found in the Doushantuo
lagerstätte, the cnidarian and bilaterian lineages must have diverged
well over 580 million years ago.[111]
Trace fossils[66] and predatory borings in
CloudinaCloudina shells provide
further evidence of
EdiacaranEdiacaran animals.[112] Some fossils from the
Doushantuo formation have been interpreted as embryos and one
(Vernanimalcula) as a bilaterian coelomate, although these
interpretations are not universally accepted.[56][57][113] Earlier
still, predatory pressure has acted on stromatolites and acritarchs
since around 1,250 million years ago.[51]
Some say that the evolutionary change was accomplished by an order of
magnitude,[note 4] but the presence of Precambrian animals somewhat
dampens the "bang" of the explosion; not only was the appearance of
animals gradual, but their evolutionary radiation ("diversification")
may also not have been as rapid as once thought. Indeed, statistical
analysis shows that the
CambrianCambrian explosion was no faster than any of
the other radiations in animals' history.[note 5] However, it does
seem that some innovations linked to the explosion – such as
resistant armour – only evolved once in the animal lineage;
this makes a lengthy Precambrian animal lineage harder to defend.[115]
Further, the conventional view that all the phyla arose in the
CambrianCambrian is flawed; while the phyla may have diversified in this time
period, representatives of the crown groups of many phyla do not
appear until much later in the Phanerozoic.[11] Further, the
mineralised phyla that form the basis of the fossil record may not be
representative of other phyla, since most mineralised phyla originated
in a benthic setting. The fossil record is consistent with a Cambrian
explosion that was limited to the benthos, with pelagic phyla evolving
much later.[11]
Ecological complexity among marine animals increased in the Cambrian,
as well later in the Ordovician.[10] However, recent research has
overthrown the once-popular idea that disparity was exceptionally high
throughout the Cambrian, before subsequently decreasing.[116] In fact,
disparity remains relatively low throughout the Cambrian, with modern
levels of disparity only attained after the early Ordovician
radiation.[10]
The diversity of many
CambrianCambrian assemblages is similar to
today's,[117][99] and at a high (class/phylum) level, diversity is
thought by some to have risen relatively smoothly through the
Cambrian, stabilizing somewhat in the Ordovician.[118] This
interpretation, however, glosses over the astonishing and fundamental
pattern of basal polytomy and phylogenetic telescoping at or near the
CambrianCambrian boundary, as seen in most major animal lineages.[119] Thus
Harry Blackmore Whittington's questions regarding the abrupt nature of
the
CambrianCambrian explosion remain, and have yet to be satisfactorily
answered.[120]
Possible causes[edit]
Despite the evidence that moderately complex animals (triploblastic
bilaterians) existed before and possibly long before the start of the
Cambrian, it seems that the pace of evolution was exceptionally fast
in the early Cambrian. Possible explanations for this fall into three
broad categories: environmental, developmental, and ecological
changes. Any explanation must explain both the timing and magnitude of
the explosion.
Changes in the environment[edit]
Increase in oxygen levels[edit]
Earth's earliest atmosphere contained no free oxygen (O2); the oxygen
that animals breathe today, both in the air and dissolved in water, is
the product of billions of years of photosynthesis. Cyanobacteria were
the first organisms to evolve the ability to photosynthesize,
introducing a steady supply of oxygen into the environment.[121]
Initially, oxygen levels did not increase substantially in the
atmosphere.[122] The oxygen quickly reacted with iron and other
minerals in the surrounding rock and ocean water. Once a saturation
point was reached for the reactions in rock and water, oxygen was able
to exist as a gas in its diatomic form.
OxygenOxygen levels in the
atmosphere increased substantially afterward.[123] As a general trend,
the concentration of oxygen in the atmosphere has risen gradually over
about the last 2.5 billion years.[20]
OxygenOxygen levels seem to have a positive correlation with diversity in
eukaryotes well before the
CambrianCambrian period.[124] The last common
ancestor of all extant eukaryotes is thought to have lived around 1.8
billion years ago. Around 800 million years ago, there was a notable
increase in the complexity and number of eukaryotes species in the
fossil record.[124] Before the spike in diversity, eukaryotes are
thought to have lived in highly sulfuric environments. Sulfide
interferes with mitochondrial function in aerobic organisms, limiting
the amount of oxygen that could be used to drive metabolism. Oceanic
sulfide levels decreased around 800 million years ago, which supports
the importance of oxygen in eukaryotic diversity.[124]
The shortage of oxygen might well have prevented the rise of large,
complex animals. The amount of oxygen an animal can absorb is largely
determined by the area of its oxygen-absorbing surfaces (lungs and
gills in the most complex animals; the skin in less complex ones);
but, the amount needed is determined by its volume, which grows faster
than the oxygen-absorbing area if an animal's size increases equally
in all directions. An increase in the concentration of oxygen in air
or water would increase the size to which an organism could grow
without its tissues becoming starved of oxygen. However, members of
the
Ediacara biotaEdiacara biota reached metres in length tens of millions of years
before the
CambrianCambrian explosion.[39] Other metabolic functions may have
been inhibited by lack of oxygen, for example the construction of
tissue such as collagen, required for the construction of complex
structures,[125] or to form molecules for the construction of a hard
exoskeleton.[126] However, animals are not affected when similar
oceanographic conditions occur in the Phanerozoic; there is no
convincing correlation between oxygen levels and evolution, so oxygen
may have been no more a prerequisite to complex life than liquid water
or primary productivity.[127]
Ozone formation[edit]
The amount of ozone (O3) required to shield Earth from biologically
lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is
believed to have been in existence around the
CambrianCambrian explosion.[128]
The presence of the ozone layer may have enabled the development of
complex life and life on land, as opposed to life being restricted in
the water.
Snowball Earth[edit]
Main article: Snowball Earth
In the late
Neoproterozoic (extending into the early Ediacaran
period), the Earth suffered massive glaciations in which most of its
surface was covered by ice. This may have caused a mass extinction,
creating a genetic bottleneck; the resulting diversification may have
given rise to the Ediacara biota, which appears soon after the last
"Snowball Earth" episode.[129] However, the snowball episodes occurred
a long time before the start of the Cambrian, and it is hard to see
how so much diversity could have been caused by even a series of
bottlenecks;[41] the cold periods may even have delayed the evolution
of large size organisms.[51]
Increase in the calcium concentration of the
CambrianCambrian seawater[edit]
Newer research suggests that volcanically active midocean ridges
caused a massive and sudden surge of the calcium concentration in the
oceans, making it possible for marine organisms to build skeletons and
hard body parts.[130] Alternatively a high influx of ions could have
been provided by the widespread erosion that produced Powell's Great
Unconformity.[131]
An increase of calcium may also have been caused by erosion of the
Transgondwanan Supermountain that existed at the time the explosion.
The roots of the mountain are preserved in present-day
East AfricaEast Africa as
an orogen.[132]
Developmental explanations[edit]
Further information: Evolutionary developmental biology
A range of theories are based on the concept that minor modifications
to animals' development as they grow from embryo to adult may have
been able to cause very large changes in the final adult form. The Hox
genes, for example, control which organs individual regions of an
embryo will develop into. For instance, if a certain
Hox geneHox gene is
expressed, a region will develop into a limb; if a different Hox gene
is expressed in that region (a minor change), it could develop into an
eye instead (a phenotypically major change).
Such a system allows a large range of disparity to appear from a
limited set of genes, but such theories linking this with the
explosion struggle to explain why the origin of such a development
system should by itself lead to increased diversity or disparity.
Evidence of Precambrian metazoans[41] combines with molecular
data[133] to show that much of the genetic architecture that could
feasibly have played a role in the explosion was already well
established by the Cambrian.
This apparent paradox is addressed in a theory that focuses on the
physics of development. It is proposed that the emergence of simple
multicellular forms provided a changed context and spatial scale in
which novel physical processes and effects were mobilized by the
products of genes that had previously evolved to serve unicellular
functions. Morphological complexity (layers, segments, lumens,
appendages) arose, in this view, by self-organization.[134]
Horizontal gene transferHorizontal gene transfer has also been identified as a possible factor
in the rapid acquisition of the biochemical capability of
biomineralization among organisms during this period, based on
evidence that the gene for a critical protein in the process was
originally transferred from a bacterium into sponges.[135]
Ecological explanations[edit]
These focus on the interactions between different types of organism.
Some of these hypotheses deal with changes in the food chain; some
suggest arms races between predators and prey, and others focus on the
more general mechanisms of coevolution. Such theories are well suited
to explaining why there was a rapid increase in both disparity and
diversity, but they must explain why the "explosion" happened when it
did.[41]
End-
EdiacaranEdiacaran mass extinction[edit]
Main article: End-
EdiacaranEdiacaran extinction
Evidence for such an extinction includes the disappearance from the
fossil record of the
Ediacara biotaEdiacara biota and shelly fossils such as
Cloudina, and the accompanying perturbation in the δ13C record.
Mass extinctions are often followed by adaptive radiations as existing
clades expand to occupy the ecospace emptied by the extinction.
However, once the dust had settled, overall disparity and diversity
returned to the pre-extinction level in each of the Phanerozoic
extinctions.[41]
Evolution of eyes[edit]
Main article: Evolution of the eye
Andrew Parker has proposed that predator-prey relationships changed
dramatically after eyesight evolved. Prior to that time, hunting and
evading were both close-range affairs – smell, vibration, and
touch were the only senses used. When predators could see their prey
from a distance, new defensive strategies were needed. Armor, spines,
and similar defenses may also have evolved in response to vision. He
further observed that, where animals lose vision in unlighted
environments such as caves, diversity of animal forms tends to
decrease.[136] Nevertheless, many scientists doubt that vision could
have caused the explosion. Eyes may well have evolved long before the
start of the Cambrian.[137] It is also difficult to understand why the
evolution of eyesight would have caused an explosion, since other
senses, such as smell and pressure detection, can detect things at a
greater distance in the sea than sight can; but the appearance of
these other senses apparently did not cause an evolutionary
explosion.[41]
Arms races between predators and prey[edit]
The ability to avoid or recover from predation often makes the
difference between life and death, and is therefore one of the
strongest components of natural selection. The pressure to adapt is
stronger on the prey than on the predator: if the predator fails to
win a contest, it loses a meal; if the prey is the loser, it loses its
life.[138]
But, there is evidence that predation was rife long before the start
of the Cambrian, for example in the increasingly spiny forms of
acritarchs, the holes drilled in
CloudinaCloudina shells, and traces of
burrowing to avoid predators. Hence, it is unlikely that the
appearance of predation was the trigger for the
CambrianCambrian "explosion",
although it may well have exhibited a strong influence on the body
forms that the "explosion" produced.[51] However, the intensity of
predation does appear to have increased dramatically during the
Cambrian[139] as new predatory "tactics" (such as shell-crushing)
emerged.[140] This rise of predation during the
CambrianCambrian was confirmed
by the temporal pattern of the median predator ratio at the scale of
genus, in fossil communities covering the
CambrianCambrian and Ordovician
periods, but this pattern is not correlated to diversification
rate.[141] This lack of correlation between predator ratio and
diversification over the
CambrianCambrian and
OrdovicianOrdovician suggests that
predators did not trigger the large evolutionary radiation of animals
during this interval. Thus the role of predators as triggerers of
diversification may have been limited to the very beginning of the
"
CambrianCambrian explosion".[141]
Increase in size and diversity of planktonic animals[edit]
GeochemicalGeochemical evidence strongly indicates that the total mass of
plankton has been similar to modern levels since early in the
Proterozoic. Before the start of the Cambrian, their corpses and
droppings were too small to fall quickly towards the seabed, since
their drag was about the same as their weight. This meant they were
destroyed by scavengers or by chemical processes before they reached
the sea floor.[34]
Mesozooplankton are plankton of a larger size. Early Cambrian
specimens filtered microscopic plankton from the seawater. These
larger organisms would have produced droppings and corpses that were
large enough to fall fairly quickly. This provided a new supply of
energy and nutrients to the mid-levels and bottoms of the seas, which
opened up a huge range of new possible ways of life. If any of these
remains sank uneaten to the sea floor they could be buried; this would
have taken some carbon out of circulation, resulting in an increase in
the concentration of breathable oxygen in the seas (carbon readily
combines with oxygen).[34]
The initial herbivorous mesozooplankton were probably larvae of
benthic (seafloor) animals. A larval stage was probably an
evolutionary innovation driven by the increasing level of predation at
the seafloor during the
EdiacaranEdiacaran period.[9][142]
Metazoans have an amazing ability to increase diversity through
coevolution.[53] This means that an organism's traits can lead to
traits evolving in other organisms; a number of responses are
possible, and a different species can potentially emerge from each
one. As a simple example, the evolution of predation may have caused
one organism to develop a defence, while another developed motion to
flee. This would cause the predator lineage to split into two species:
one that was good at chasing prey, and another that was good at
breaking through defences. Actual coevolution is somewhat more subtle,
but, in this fashion, great diversity can arise: three quarters of
living species are animals, and most of the rest have formed by
coevolution with animals.[53]
Ecosystem engineering[edit]
Evolving organisms inevitably change the environment they evolve in.
The
DevonianDevonian colonization of land had planet-wide consequences for
sediment cycling and ocean nutrients, and was likely linked to the
DevonianDevonian mass extinction. A similar process may have occurred on
smaller scales in the oceans, with, for example, the sponges filtering
particles from the water and depositing them in the mud in a more
digestible form; or burrowing organisms making previously unavailable
resources available for other organisms.[143]
Complexity threshold[edit]
The explosion may not have been a significant evolutionary event. It
may represent a threshold being crossed: for example a threshold in
genetic complexity that allowed a vast range of morphological forms to
be employed.[144] This genetic threshold may have a correlation to the
amount of oxygen available to organisms. Using oxygen for metabolism
produces much more energy than anaerobic processes. Organisms that use
more oxygen have the opportunity to produce more complex proteins,
providing a template for further evolution.[122] These proteins
translate into larger, more complex structures that allow organisms
better to adapt to their environments.[145] With the help of oxygen,
genes that code for these proteins could contribute to the expression
of complex traits more efficiently. Access to a wider range of
structures and functions would allow organisms to evolve in different
directions, increasing the number of niches that could be inhabited.
Furthermore, organisms had the opportunity to become more specialized
in their own niches.[145]
Uniqueness of the explosion[edit]
The "
CambrianCambrian explosion" can be viewed as two waves of metazoan
expansion into empty niches: first, a coevolutionary rise in diversity
as animals explored niches on the
EdiacaranEdiacaran sea floor, followed by a
second expansion in the early
CambrianCambrian as they became established in
the water column.[53] The rate of diversification seen in the Cambrian
phase of the explosion is unparalleled among marine animals: it
affected all metazoan clades of which
CambrianCambrian fossils have been
found. Later radiations, such as those of fish in the
SilurianSilurian and
DevonianDevonian periods, involved fewer taxa, mainly with very similar body
plans.[20] Although the recovery from the Permian-Triassic extinction
started with about as few animal species as the
CambrianCambrian explosion,
the recovery produced far fewer significantly new types of
animals.[146]
Whatever triggered the early
CambrianCambrian diversification opened up an
exceptionally wide range of previously unavailable ecological niches.
When these were all occupied, limited space existed for such
wide-ranging diversifications to occur again, because strong
competition existed in all niches and incumbents usually had the
advantage. If a wide range of empty niches had continued, clades would
be able to continue diversifying and become disparate enough for us to
recognise them as different phyla; when niches are filled, lineages
will continue to resemble one another long after they diverge, as
limited opportunity exists for them to change their life-styles and
forms.[147]
There were two similar explosions in the evolution of land plants:
after a cryptic history beginning about 450 million years ago,
land plants underwent a uniquely rapid adaptive radiation during the
DevonianDevonian period, about 400 million years ago.[20] Furthermore,
Angiosperms (flowering plants) originated and rapidly diversified
during the Cretaceous period.
See also[edit]

^ This included at least animals, phytoplankton and calcimicrobes.[9]
^ At 610 million years ago,
AspidellaAspidella disks appeared, but it is not
clear that these represented complex life forms.
^ Older marks found in billion-year-old rocks[59] have since been
recognised as nonbiogenic.[11][60]
^ As defined in terms of the extinction and origination rate of
species.[53]
^ The analysis considered the bioprovinciality of trilobite lineages,
as well as their evolutionary rate.[114]

The
CambrianCambrian "explosion" of metazoans and molecular biology: would
Darwin be satisfied?
On embryos and ancestors by Stephen Jay Gould
Conway Morris, S. (April 2000). "The
CambrianCambrian "explosion": Slow-fuse
or megatonnage?". Proceedings of the National Academy of Sciences. 97:
4426–4429. Bibcode:2000PNAS...97.4426C. doi:10.1073/pnas.97.9.4426.
PMC 34314 . PMID 10781036.
The
CambrianCambrian Explosion – In Our Time,
BBC Radio 4BBC Radio 4 broadcast, 17
February 2005
"Burgess Shale". Virtual Museum of Canada. 2011. , exhaustive
details about the Burgess Shale, its fossils, and its significance for
the
CambrianCambrian explosion
Utah's
CambrianCambrian life – new (2008) website with good images of a
range of Burgess-shale-type and other
CambrianCambrian fossils
Smithsoni